This relates to solid-state image sensor arrays and, more particularly, to image sensors with optically black pixels for compensating for dark current and electrical non-uniformities generated by image sensors.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels. The image pixels generate image signals in multiple color channels in response to light that is received at the image sensor. Readout circuitry such as analog-to-digital converter circuits are commonly coupled to the image pixels for reading out image signals from the image pixels.
The image pixels in an image sensor may generate signals even when no light is incident upon the image sensor (i.e., image pixels may generate a background signal). Signals that are generated in the absence of incident light may cause noise (i.e., fixed pattern noise) in the image signals generated by the sensor in response to light that is received at the image sensor. Noise generated by the image pixels in an image sensor may reduce the sensitivity of the image sensor and result in an image having undesirable brightness, color, and contrast characteristics.
In an image sensor, the temperature of the image sensor substrate typically affects the image signals generated by the image pixels. In a typical pixel photodiode, there is some current (i.e., dark current) in the photodiode even when no light is incident upon the photodiode (due to the inherent movement of electrons across the corresponding semiconductor junction). As the temperature of the photodiode increases, this flow of electrons, and therefore the dark current, increases. Increased dark current in the image sensor can cause excessive and unsightly noise in the final image signal.
Image pixels in different locations on a pixel array may generate different levels of dark current and background noise. For example, because the level of dark current generated by a photodiode is temperature-dependent, image pixels that are located in a portion of a pixel array that has a higher temperature (i.e., near image sensor circuitry that generates heat) may have a higher temperature and generate more dark current or noise than image pixels that are in an area of the pixel array with a lower temperature. The temperature may vary across an image sensor to form a temperature gradient. Likewise, the level of dark current and/or noise present in the signals generated by pixels located in different locations in the pixel array may form a gradient based on the location of the image pixels in the array. It is therefore useful to be able to determine the level of dark current and noise produced by the image pixels in the image sensor array so that dark current contributions to the final image signal can be compensated for.
One method of compensating for dark current and noise generated by the pixels in an image sensor array involves replacing a conventional an image sensor pixel with a “dummy” pixel that only produces a dark current signal and does not produce a photocurrent signal (i.e., an image signal generated in response to light received at the pixel). This dark current signal may be used to generate a correction value that can be used to correct for the dark current produced by the normal image sensor pixels in the vicinity of the dummy pixel. However, this method of dark current compensation requires knocking out pixels in the active portion of the image sensor array, and may result in images having dead pixels and low resolution. If the resolution of the image sensor is to be maintained, only a limited number of image sensor pixels may be knocked out. However, this leads to limited locations at which the dark current/noise contributions of the image pixels in the array can be sensed, and results in correction values that may not be able to provide accurate dark current compensation values for all pixels in the array (e.g., for pixels that are not located near a dummy pixel).
It would therefore be desirable to provide imaging devices with improved systems and methods for compensating for dark current and electrical non-uniformities in image signals generated by image sensors.